![]() ![]() Figure 10.16 “Liquids and Gravity.” (a) A liquid fills the bottom of its container as it is drawn downward by gravity and the particles slide over each other. This explains why liquids can assume the shape of their containers: the particles move around and, under the influence of gravity, fill the lowest volume possible (unless the liquid is in a zero-gravity environment - see Figure 10.16 “Liquids and Gravity”). This is the liquid phase: particles are still in contact but are able to move around each other. When more energy is supplied (e.g., by raising the temperature), there comes a point at which the particles have enough energy to move around but not enough energy to separate. What happens when a solid becomes a liquid? In a solid, individual particles are stuck in place because the intermolecular forces cannot be overcome by the energy of the particles. Only when all of a substance is melted does any additional energy go to changing its temperature. Hence melting is an isothermal process because a substance stays at the same temperature. ![]() What is the energy change when 108 g of C 6H 6 freeze at 5.5☌?ĭuring melting, energy goes exclusively to changing the phase of a substance it does not go into changing the temperature of a substance. Without a sign, the number is assumed to be positive. Because the substance is melting, the process is endothermic, so the energy change will have a positive sign. Then we can use Δ H fus as a conversion factor. However, our quantity is given in units of grams, not moles, so the first step is to convert grams to moles using the molar mass of H 2O, which is 18.0 g/mol. What is the energy change when 45.7 g of H 2O melt at 0☌? Table 10.2 Enthalpies of Fusion for Various Substances Substance (Melting Point) However, it can be used for both the melting and the solidification processes as long as you keep in mind that melting is always endothermic (so Δ H will be positive), while solidification is always exothermic (so Δ H will be negative). The Δ H fus is always tabulated as a positive number. Note that the unit of Δ H fus is kilojoules per mole, so we need to know the quantity of material to know how much energy is involved. Some Δ H fus values are listed in Table 10.2 “Enthalpies of Fusion for Various Substances” it is assumed that these values are for the melting point of the substance. This amount is called the enthalpy of fusion (or heat of fusion) of the substance, represented as Δ H fus. Every pure substance has a certain amount of energy it needs to change from a solid to a liquid. It requires energy for a solid to melt into a liquid. For any pure substance, the temperature at which melting occurs - known as the melting point - is a characteristic of that substance. ![]() The opposite process, a liquid becoming a solid, is called solidification. The process of a solid becoming a liquid is called melting (an older term that you may see sometimes is fusion). At low temperatures, most substances are solid as the temperature increases, they become liquid at higher temperatures still, they become gaseous. Substances can change phase - often because of a temperature change.
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